This invention relates to a method for preparing transparent methyl-modified silica gel using the sol-gel technology. Particularly, it relates to the preparation of transparent methyl-modified silica gel in bulk, film, sheet or other forms useful as a host material for optically functional organic molecules.
Glasses and ceramics produced by the sol-gel technology are susceptible to a remarkable volumetric shrinkage and therefore cracking upon drying due to the evaporation of water or alcohol contained in the gel. Several solutions are known for these problems but they are successful only with difficulties in producing transparent gel because the gel is porous in nature.
Thin films of a glass or ceramic produced by the sol-gel process may be formed on a substrate using the dip- or spin-coating method. Because the film thickness achievable by this process at one time of application is normally limited to only about 1.5 .mu.m or less, it is imperative to repeat the coating and sintering operations when thicker films are desired. This process suffers from the problems of volumetric shrinkage and therefore generation of cracks and debonding.
JP-A-04362033 teaches a method for producing relatively thick silica gel films. The method includes the steps of spreading a solution of alkoxysilane in an organic solvent on the surface of an aqueous solution containing a base to hydrolyze and polycondensate the alkoxysilane at the interface between the aqueous phase and the organic phase, taking up the resultant film on a substrate, and heating the film at an elevated temperature. JP-A-62247834 teaches a method for producing flaky ceramic particles from a sol including the step of applying the sol on a support sheet followed by gelling. JP-A-61236620 and JP-A-62070237 disclose a method for producing silica gel film including the step of casting silica sol on the surface of a liquid having a large specific gravity such as mercury, dibromoethane or tristearin.
The films produced by these methods are also porous in nature and require a heating step at a temperature of 1,000 .degree. C. or higher to obtain a dense film, resulting in remarkable volumetric shrinkage, warps and cracks. Accordingly, it is difficult to produce a self-sustained film made of three-dimensional silica based network having a thickness from about 30.mu.m to 200.mu.m by the prior art methods.
It is known in the literature to synthesize silica gel doped with optically functional organic molecules by the sol-gel process. Tani et al., J. Appl. Phys., 58 (1985), 3559; and Avnir et al., J. Phy. Chem., 88 (1984), 5956. However, the gel possesses a well-developed three-dimensional network structure and the space defined by the silica network is not large enough in size to receive high concentration of organic substances having a relatively large molecular size. It has been studied to enlarge the space defined by the silica network either by introducing organic groups into the silica network or by complexing silica with an organic polymer. However, it is difficult to produce silica gel-based, shaped articles having satisfactory mechanical, optical, chemical and thermal properties.
It is known that precipitation and phase separation are often experienced in the sol-gel process due to rapid hydrolysis reaction or poor solubility of the hydrolysate. These phenomena have also been observed when preparing methyl-modified silica gel coatings. Accordingly, it is critical for the production of silica gel-based optical materials by means of sol-gel technology to inhibit the formation of precipitates and the occurrence of phase separation.
Therefore, a need exists for a method of production of transparent methyl-modified silica gel and various shaped articles made therefrom, which may eliminate or ameliorate various problems associated with the above-discussed prior art methods.